It is well recognized that the graphic arts printing market desires, at this time, a high-speed digital press. A digital press that begins to match the speed, image quality, and per print costs of conventional printing presses would complement the digital nature of information and enable variable data printing. Several electrophotographic-based engines exist today, with both dry and liquid toning systems. The dry toning systems suffer from image relief, limited process width, generally high print costs, low process speed, and high process complexity. The liquid based systems suffer from limited process width, and a complex process, which requires sophisticated operation.
Ink jet printing has been touted as a technology of choice for digital printing, but also has several problems. Even assuming the successful development of full-width printheads, aqueous-based ink jet inks, being approximately 95% water, struggle to achieve high densities in a single pass, soak the receiver (e.g., paper) inducing cockle and additional drying costs, and are subject to coalescence problems, worsened by the full-width, single-pass printing mode required to achieve press-like throughput.
The problem of coalescence is particularly troublesome when attempting high speed printing via ink jet. If ink drops on the receiver touch one another, surface tension causes them to pool into a blob, destroying the spatial integrity of the image. Several patents have addressed the problem, if even as a means to solve other problems, by jetting onto patterned surfaces. U.S. Pat. No. 6,109,746 (Jeanmaire et al.), jets onto a patterned surface; as does U.S. Pat. No. 6,443,571 (Shinkoda et al.); and U.S. Pat. No. 6,648,470 (Korem). All of these systems are aqueous based, however, and retain the density problem and add a new one: residual colorant left in the cellular structure from incomplete transfer.
Transferring ink from the cell of a patterned surface to a receiver is akin to conventional transfer in, say, a gravure printing press. Press inks transfer at 50-60% efficiency, but the residual ink is simply refreshed (the cell is refilled) and the same image printed again in register (to the other colors). A digital press, however, with fully variable printing capability, requires that each image be potentially different and thus cleaning or removal of the residual ink is required. Assuming one could clean the cells of the residual ink (a very difficult task at high speed), one could not simply discard it, since this would essentially double the ink costs of printing, a generally unattractive proposal for printers.
It is the object of this invention to provide a process that enables fully variably digital printing at high speeds while simultaneously overcoming the problems of coalescence, adequate single-pass density, excessive water volume on the receiver, and residual colorant in the cells of a patterned ink-receiving surface.